X-ray imaging at the dentist takes one of three major forms: static images, mostly done as a part of the dental checkup (“status”), panoramic imaging, where a panoramic view of the whole mouth results and “cone-beam computerized tomography” (CBCT) imaging, where volumetric information of the mouth is collected, reconstructed and displayed.
Static imaging is ordinarily performed using the clinics' x-ray source and intraoral film or an intraoral digital flat panel detector (FPD). The three most used scan geometries are: Bitewing scan, which shows the crown portions of the top and bottom teeth together; Periapical scan, which shows one or two complete teeth from crown to root, and Palatal or Occlusal scan, which shows the roof or floor of the mouth. In Bitewing and in Periapical scans the dentist inserts the radiographic film or FPD into the patient's mouth and fixes it behind the area of interest (ROI). Then, the dentist approaches the x-ray source, mounts it externally against the ROI and shoots the x-ray. The film then has to be developed, or the information from the FPD be uploaded to a host system for display.
A more sophisticated x-ray system is used for panoramic imaging. It has a rotating arm, carrying the x-ray source on one end and the FPD on the other end (the following assumes that FPD is used, not a film). The patient's head is positioned within chin, forehead and side rests, and is coaxial to the rotating arm. The patient is also provided with a bite blocker to prop open the oral cavity. Exposure is performed while the source and the FPD continuously rotate around the patient head. A set of discrete images is thus obtained, which capture different sections of the mouth. Next, a specialized algorithm stitches the images together, resulting a single, flattened image of the mouth.
CBCT imaging is done in a similar manner, where the system takes many “views” around the patient. However, in this case, it reconstructs the measured data, thus resulting in a 3-dimensional representation of the mouth.
An important factor of any imaging modality is the spatial resolution. The resolution depends both upon the size of the radiation source (as expressed in the beam's spot-size) and the image sampling spacing (in FPD it is the pixel pitch). The various methods of static imaging all require high resolution (for visualizing the fine details of the gums and teeth)—typically of about 30 μm size. For panoramic imaging the resolution is typically 150 μm, and for CBCT it is about 250 μm. A film is sometimes still being used for static imaging, because films have an intrinsically high spatial resolution.
Alternatively, if an intraoral FPD is used, it must have the required pixel pitch of about 30 μm, comparable with film resolution. For panoramic scans, the source size is about 300 μm, while the FPD pixel is about 100 μm. Therefore, the FPD is kept closer to the patient head, for bringing the resolution closer to the required 150 μm. CBCT utilizes the same source and FPD as in the panoramic imaging system; however, the FPD can be mounted farther away from the patient and the resolution is, accordingly, lower.
While CBCT is considered the “high-end” of the dentistry imaging and is used only for full mouth reconstruction, the static and panoramic imaging methods are much more common. Yet, they suffer from major technological drawbacks: Static imaging requires multiple exposures. For example, four bitewing scans and 10-17 Periapical scans are taken in a typical checkup meeting. The mounting in and out of the imager is unpleasant both to the patient and to the dentist. In addition, the whole process is time consuming, which includes the times of fixing the FPD inside the patient's mouth, mounting the x-ray tube, moving behind an x-ray shield, performing the exposure and reading the resulting image.
Panoramic viewing is faster and more convenient. However, the method of encompassing the patient's head necessitates radiation exposure to extraoral tissues, notably the patient's spine. Therefore, the absorbed dose for the patient is considerably higher than required for sheer imaging of the mouth. Furthermore, the image quality deteriorates, because the spine casts a shadow on the front teeth. A higher voltage x-ray setting (typically 80 kVp and more) is also required in order for the beam to pass through the spine, which reduces the contrast between the teeth/bones and the soft tissues (gums). Yet a further problem is that the magnifications of the images vary as the system performs the scan, because both the source-to-tooth-distance and the FPD-to-tooth-distance change as the FPD and tube move around the patient head. Therefore, neighboring images have different magnifications on their edges, resulting in blurring of the stitched image.
One partial solution to the panoramic drawbacks is described in U.S. Pat. No. 4,176,278. An intraoral x-ray source based upon an encapsulated nuclear source is disclosed, which is connected through a gears-and-pulleys system to a film holder. The device fixes the magnification, thus eliminating the need in moving the source and the patient head. However, the use of a nuclear source necessitates arrangements of storing and transferring it within the clinic. Furthermore, the sources decay and require frequent replacements. The complicated mechanical device makes the filming apparatus cumbersome and expensive and makes the filming itself much more difficult. Finally, none of the problems of static imaging are solved.
US 20130010923 describes another intraoral radiation type X-ray imaging system for capturing an X-ray image of teeth or a jawbone that includes: a tubular X-ray generating unit (having a electron-emitting source, an X-ray emitting source and a collimator) placed in the oral cavity; a high-voltage source providing power for the X-ray generating unit; an X-ray detection unit, and an X-ray generating unit support to control the position of the X-ray generating unit in the oral cavity.
In US 20130010923 the high-voltage source is described as positioned outside of the mouth. This setup requires high-voltage cabling that goes between the jaws, making the important Bitewing (i.e., teeth pressed together) imaging impossible. In addition, even the option of threading the cabling through the unit support, as described therein, might also pose a safety issue, and is certainly a cause for concern, since the 100 kVp cables are being bitten and moistened by the patient. Moreover, since the collimator absorbs much of the X-ray radiation, the required current is as high as 15 mA.
Also described therein is rotation of the detector around the mouth, the intraoral source following the detector. However, the source is mounted on a shaft going from the inside to the outside of the patient mouth. This arrangement also disallows any imaging with teeth pressed together.
The described x-ray source emits the radiation to all directions (more than) 180°. Accordingly, for panoramic scans, the angular acceptance may be limited by a collimator that is possibly rotated to the desired orientation. However, this arrangement is also impractical because the emitted radiation has limited emitting angle, and it is also absorbed differently by the X-ray emitting source, depending on the particular orientation. Alternatively, the detector moves and the X-ray unit repeatedly emits X-rays the entire region of examined teeth, which constitutes superfluous exposure to radiation.
In addition, X-ray generating unit must be exactly placed such that an affected part of the patient is appropriately imaged with a minimal dose of radiation. Such placing is difficult to perform.
US 20130010923 further discloses that when the electron source to X-ray source distance exceeds 3 cm, it is difficult to place the X-ray generating unit in the oral cavity, and electrons does not exactly reach the X-ray generating source, and thus a sufficient dose of X-ray radiation is not radiated, thereby deteriorating the quality of X-ray images obtained. Therefore, in order to achieve the miniaturization of the X-ray generating unit placed in the oral cavity, the X-ray generating source, the electron emitting source and a power connection unit are disposed on the same straight line axis, and when the elements are disposed on the same straight line, effects of a high voltage power line applied from the outside on the movement of electrons emitted from the electron emitting source are minimally exerted, thereby improving the stability of operating the X-ray generating device. However, making said alignment is also technically very difficult to perform.
An object of the invention is to provide solutions for the above concerns.